This Bug seems to be portal exclusive/only possible when enemies have elements tied to them.
Based on my testing, this bug occurs when multiple elemental reactions that result in stun happen at the same time.
In this case, the enemies have the elements of Fire (main focus) and Water (not so relevant).
As you can see in the first video, Esdeath gets infinitely stunned when vergil (spark) and gogeta (blast) attack/kill an enemy, resulting in both of their respective elemental reactions to go off.
In the next video, as I move gogeta closer in order to further test the bug, you see that as both units attack and the reaction happens (diavolo avoids the explosion, triggering his counter attack passive) vergil and gogeta are then infinitely stunned as well
